1. Field of the Invention
The present invention relates to (Ba,Sr)TiO3 (BST) thin films, and more particularly to a method for creating a haze-free BST thin film with a high dielectric constant.
2. Description of the Related Art
(Ba,Sr)TiO3 (BST) films are commonly used as dielectric materials for capacitors, gate dielectrics and high frequency electronic circuits. More particularly, BST films have found application as capacitors in dynamic random access memory (DRAM) cells. A typical DRAM cell comprises a charge storage capacitor (or cell capacitor) coupled to an access device such as a metal-oxide semiconductor field effect transistor (MOSFET). The MOSFET functions to apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is determined by the capacitance, C=∈∈0A/d, where ∈ is the dielectric constant of the capacitor dielectric, ∈0 is the vacuum permittivity, A is the electrode (or storage node) area, and d is the interelectrode spacing. The conditions of DRAM operation such as operating voltage, leakage rate and refresh rate, will in general mandate that a certain minimum charge be stored by the capacitor.
BST is desirable for such applications because of its high dielectric constant, low DC leakage, low dispersion up to high frequencies and stable operation at high temperatures. The high dielectric constant of BST thereby gives the material the ability to yield high capacitance when placed between a pair of electrodes. BST films grown for applications such as DRAM capacitors are typically made using metal-organic chemical vapor deposition (MOCVD) or sputtering. However, MOCVD growth of such films typically leads to problems such as haze which reduces the dielectric constant of the material and increases leakage currents. Specifically, haze is caused by the growth of spatially correlated, non-textured BST, which in turn creates discernible optical scatter and a cloudy or hazy appearance in the film. For example, haze may be created when a film desired to be grown in a (100) orientation has orientations other than (100), such as (110) or (111), thereby disrupting the texture of the film. When BST is used in a capacitor structure, haze causes its capacitance to decrease as much as 50% and leakage currents to increase by a factor of 10 to 1000 with respect to smooth films.
Furthermore, electrodes and other materials on which BST films are deposited often suffer from process-induced defects such as hillock formation which may severely limit performance. Hillocks are small nodules which form when the electrode or other material is deposited or subjected to post-deposition processing. For example, hillocks can result from excessive compressive stress induced by the difference in thermal expansion coefficient between the BST film and the underlying electrode material during post-deposition heating steps. Such thermal processing is typical in the course of semiconductor fabrication. Hillock formation may create troughs, breaks, voids and spikes along the electrode surface, thereby leading to uneven BST growth and stress in the BST film.
Accordingly, it is an object of this invention to produce a haze-free BST thin film with a high dielectric constant and low leakage currents. It is also an object of this invention to produce a BST thin film that has low stress. It is further an object of this invention to produce an electrode or other base material onto which a BST film is deposited that is smooth and hillock-free to improve the properties of the subsequently deposited BST film.
These objects are achieved generally through the control of one or more processing conditions in the fabrication of the BST film. Briefly stated, haze can be reduced by increasing the BST deposition temperature, decreasing the deposition rate and increasing the atomic percent of titanium in the BST film. These conditions favor the formation of a highly textured film, i.e., a film with a substantially uniform desired crystal orientation. Furthermore, use of highly textured substrates, bottom electrodes or nucleation layers also favors haze-free and low stress BST films. Moreover, the above-stated objects are achieved by forming a substrate layer such as a bottom electrode and the BST film in a clustered tool.
In one aspect of the present invention, a method of forming a haze-free BST film over a substrate assembly is provided. The method comprises supplying BST sources into a chamber, and inducing textured growth of the BST film over the substrate assembly in a substantially uniform desired crystal orientation. In one preferred embodiment, the BST film is deposited at a rate of less than about 80 xc3x85/min at a chamber temperature above about 580xc2x0 C. The BST film is preferably grown using metal-organic chemical vapor deposition (MOCVD), and results in a film having a concentration of about 50 to 53.5 atomic percent titanium.
In another aspect of the present invention, a substantially haze-free BST thin film is provided. The BST thin film has a textured structure with a substantially uniform crystal orientation.
In another aspect of the present invention, the method of forming the substantially haze-free BST film first comprises forming a nucleation layer over a substrate assembly. Then, the BST film is formed over the nucleation layer, the BST film being formed having a substantially uniform crystal orientation. In one preferred embodiment, the nucleation layer is NiO, and an orientation layer such as platinum is formed over the nucleation layer before forming the BST film. The orientation layer preferably has a desired crystal orientation to induce the same orientation in the subsequently formed BST film. In another preferred embodiment, the nucleation layer is made of Ti, Nb or Mn to compensate for defects in the subsequently formed BST film.
In another aspect of the present invention, a thin film structure is provided comprising a nucleation layer and a BST film over the nucleation layer having a substantially uniform crystal orientation. In one embodiment, an orientation layer is preferably provided over the nucleation layer underneath the BST film. In another embodiment, the BST film is directly on top of the nucleation layer.
In another aspect of the present invention, a method of forming a BST capacitor structure is provided. A first electrode material is formed over a substrate assembly, followed by forming a BST film over the first electrode material. The BST film being formed has a substantially uniform crystal orientation. A second electrode material is then formed over the BST film. The first electrode material is preferably formed in a vacuum at a temperature between about 500 and 550xc2x0 C., while the BST film is preferably formed at a temperature greater than about 580xc2x0 C. The BST film is preferably deposited in a vacuum chamber, with the first electrode material and the BST film formed without a vacuum break in between.
In another aspect of the present invention, a capacitor structure is provided comprising a base layer, a bottom electrode formed over the base layer, a BST film formed over the bottom electrode, and a top electrode formed over the BST film. The BST film has a substantially uniform orientation, and preferably comprises between about 50 and 53.5 atomic percent titanium. Preferably, a nucleation layer of NiO is provided between the base layer, which is preferably polysilicon, and the bottom electrode, which is preferably platinum. A second or alternative nucleation layer may be provided between the bottom electrode and the BST film, and more preferably comprises a material such as Ti, Mn or Nb.